Cancer resistance to radiation is often the cause of failure in radiotherapy; intracellular glutathione (GSH) has been implicated as one of the major causes of the resistance. GSH increases cancer resistance to radiation by terminating free radicals and reactive oxygen species (ROS) that are generated by radiation to destroy cancer cells. Extensive efforts have been made to increase cancer sensitivity to radiation through decreasing intracellular GSH. The most extensively investigated approach in reducing intracellular GSH has been the use of buthionine sulfoximine (BSO), an inhibitor of GSH synthesis. BSO has been demonstrated to effectively reduce GSH levels and increase cancer sensitivity to radiation. In this application, we propose to increase cancer sensitivity to radiation through inhibition of glutathione reductase (GR). This proposed hypothesis, that is inhibition of GR may provide an alternative and possibly more effective way to increase cancer response to radiation, is based on the fact that in terminating free radicals or ROS, GSH is oxidized to its oxidized form GSSG. GR is the enzyme responsible for the reduction of GSSG to GSH. Inhibition of GR will hamper the cell's ability to convert GSSG back to GSH to maintain intracellular GSH levels. As a result, inhibition of GR will not only reduce intracellular GSH levels but also cause accumulation of GSSG. Therefore, inhibition of GR may have more impact on cancer sensitivity to radiation than inhibition of GSH synthesis.Preliminary experiments conducted in this laboratory with OVCAR-3 ceils, a human ovarian cancer cell line with resistance to chemotherapy and radiotherapy, have shown that inhibition of GR did reduce intracellular GSH and the reduction of GSH led to an increase in the sensitivity of OVCAR-3 to melphalan, a chemotherapeutic agent. In this application, we are planning to extend the work to determine if inhibition of GR can also increase the sensitivity of OVCAR-3 to radiation. In addition, we are also planning to investigate the effect of GR inhibition on the cellular redox state and the enzyme systems responsible for terminating free radicals and ROS, since these enzyme systems will affect cancer sensitivity to radiation. This investigation will provide information on the effect of GR inhibition on cancer sensitivity to radiation, cellular redox state (via the ratio of thiol/disulfide), and enzyme systems involved in terminating radicals and ROS. The long-term objective is to develop a novel approach to increase cancer sensitivity to radiation.